The tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induces apoptosis in a variety of tumorigenic and transformed cell lines with little or no effect on normal cells. At least five receptors for TRAIL have been identified, of which two, namely DR4 (Death Receptor 4, TRAIL-R1) and DR5 (Death Receptor 5, TRAIL-R2; KILLER or TRICK 2), are capable of transducing an apoptosis signal, whereas the other three (TRAIL-R3, TRAIL-R4 and soluble OPG) serve as decoy receptors to block TRAIL-mediated apoptosis. See e.g., Ozoren and El-Deiry, Sem. Cancer Biol 13: 135 (2003); Yagita et al., Cancer Sci., 95(10): 777 (2004).
TRAIL or Apo2L is a 281 amino-acid cytotoxic ligand found integrated into the cytoplasmic membrane with the C-terminus exposed at the extracellular surface (Type II ligand) of cells. Small quantities of soluble TRAIL ligand can also be detected. TRAIL forms a homo-trimeric molecule that binds its respective receptors, initiating a cascade of signaling events.
The binding of TRAIL ligand to the receptors DR4 or DR5 initiates the extrinsic cell death pathway, resulting in the formation of death-inducing signaling complexes (DISC), which contain the adaptor FADD (Fas-activating DD) and pro-caspase 8 or pro-caspase 10. The interactions at the DISC and the activation of the downstream cascade are similar to FAS, resulting in activation of the NFκB and Jun N-terminal kinase pathways (JNK). See, e.g., Mongkolsapaya et al., Nat. Struct. Biol., 6(11): 1048 (1999); Cha et al., J. Biol. Chem., 275(40): 31171 (2000). TRAIL binding to DR4 or DR5 also results in a BID cleavage (by caspase 8 or 10), activation of mitochondria and hence activation of the intrinsic apoptosis pathway.
The ability of TRAIL ligand to preferentially induce apoptosis of tumor cells, with little or no effect on normal cells, makes it a potentially good candidate for cancer therapy. However, soluble TRAIL has been shown to induce apoptosis of normal human hepatocytes in vitro, highlighting potentially toxicity concerns. See, e.g., Jo et al., Nat Med. 6(5): 564 (2000). A more advantageous target for cancer therapy is DR5. Because DR5 requires multiple receptors to induce apoptosis of normal hepatocytes, the development of agonists against the specific DR5 responsible for induction of apoptosis is predicted to avoid this toxicity against normal cells, while retaining the ability to kill tumor cells.
Peptide and antibody agonists of DR5 have been used to induce apoptosis in cells expressing DR5. See, e.g., Li et al., J. Mol. Biol., 361, 522-536 (2006); Kajiwara et al., Biochim. Biophys. Acta 1699: 131-137 (2004); Yang et al., Cancer Cell, 5, 501-512 (2004). Agonists of DR5 have the potential to be used in cancer therapy against a wide range of cancers. For example, Lexatumumab, a monoclonal antibody against DR5, induces expression of DR5 and promotes apoptosis in a mouse model of renal cell carcinoma. Zhang et al., Cancer Lett. 251(1): 146-57 (2007). Other agonistic monoclonal antibodies against DR5, or single-chain Fv fragment against DR5, and the tumoricidal activity thereof are similarly described, e.g., in Takeda et al., Journal Exp. Medicine, 199, 437-448 (2004); Guo et al., J. Biol. Chem., 280, 41940-41952 (2005); Motoki et al., Clin. Cancer Res 11(8): 3126-35 (2005); and Ichikawa et al., Nature Medicine, 7, 954-960 (2001), Chuntharapai et al., J. Immunol. 166(8): 4891-8 (2001), Shi et al., Cancer Res., 66(24): 11946-11953 (2006).
Various DR5 specific antibodies are being developed for use in the clinic but none are yet approved. It is believed that for DR5 to be activated, multiple cross-linking antibodies are required, and issues remain for sufficient delivery of these antibodies at the site of action to achieve reproducible therapeutic effects. Therefore, a need exists for improved DR5 specific agonists for treatment of associated diseases.